1. Field of the Invention
This invention relates to auto-focus devices for video cameras and, more particularly, to a solid-state image sensing apparatus in which auto-focusing (hereinafter AF) is performed without using a drive portion for carrying out modulation of the optical path or the like to obtain focus information.
2. Description of the Related Art
The prior known auto-focus devices utilizing TV video signals are described in great detail in "Showa 40th year's NHK technology research Volume 17, No. 1, serial No. 86, pages 21-37" and others. Also, as described in Japanese Laid-Open Patent Application No. Sho 52-104245 and Japanese Laid-Open Patent Application No. Sho 56-51164, there is a so-called "hill climb servo type" in which the focusing lens is moved so that the focus voltage proportional to the magnitude of the high frequency component of the video signal gets ever higher. In another type as described in Japanese Laid-Open Patent Application No. Sho 58-188965, the image sensor or the focusing lens itself is moved to a small excursion to modulate the optical path, and from the phase of change of the focus voltage, the direction in which the focusing lens is to move is determined, so that the focusing lens is moved in the above-described way or in the direction to increase the focus voltage. Since, in the device of this so-called "optical path modulation type", information representing the near-focus state, or the far-focus state of the lens can be obtained, the direction in which the lens is to move can previously be sensed, so that the focusing can be carried out accurately and swiftly.
In the hill climb servo type described in Japanese Laid-Open Patent Application No. Sho 52-104245, however, the focusing lens must preliminarily be moved. Otherwise, the lens drive direction cannot be known. Thus, once the focusing lens is moved, if the blurring intensifies, the direction is reversed, or if the image gets sharper, the movement of the focusing lens in that direction is continued. Where the blurring no longer diminishes by moving it in either of the two directions is taken as the in-focus point. Therefore, there are many wasteful motions. Another drawback is that the focusing speed becomes slow. In the type described in Japanese Laid-Open Patent Application No. Sho 58-188965, on the other hand, because besides the movement for focus adjustment of the focusing lens, minute excursions of the image sensor, the focusing lens, etc. must be performed. For this reason, the construction of the device itself becomes complicated and large-sized. Nevertheless, the requirement for precision accuracy is rigorous. These points constitute problems of the latter type.